A long-standing hurdle, which has greatly plagued biomedical optical technologies, is the turbidity of biological tissue. Due to significant scattering and absorption loss, light cannot be efficaciously delivered to or collected from target regions within deep tissue, significantly hindering the capability to monitor post-surgical healing of tissues or organs, perform highly targeted light-based therapy, or optogenetic stimulation, to name but a few examples. Implanting fibrous optical waveguide in tissues or organs for light delivery or collection is one of the most effective methods for alleviating this problem. However, traditional silica fibers are not only non-degradable, but also fragile and brittle in nature, thus presenting a significant limitation as an implantable device. Waveguides made from single traditional materials, such as poly(ethylene glycol) (PEG), silk, agarose gel, and poly(L-lactic acid) (PLA) have also been reported. However, due to the lack of an intrinsic cladding layer, single material waveguides tend to have high loss, resulting from significant interaction of the guided optical wave with the surrounding medium (such as tissues in vivo). To address this issue, a biocompatible step-index fiber optical waveguide consisting of a PEG core and an alginate hydrogel cladding was developed for organ-scale light delivery and collection. Later, fibers having a step-index structure but made of alginate-polyacrylamide hydrogel and silk were also demonstrated. Despite the progress, hitherto the underlying materials either suffer from non-degradability or have limited processability and designability. In general, a fundamental challenge of the field is the lack of a suitable material platform that can simultaneously meet the diversified requirements on optical (tailored refractive indices for both the core and the cladding, low optical loss), mechanical (tunable mechanical flexibility for tissue compliance), and biological (biocompatibility and programmable biodegradability) functionalities.